In recent years, cellular mobile communications have evolved from the Universal Mobile Telecommunication System (UMTS) to the Long Term Evolution (LTE). The LTE uses the Orthogonal Frequency Division Multiplexing (OFDM) as a basis of its radio access technology, to achieve high-speed radio packet communications with a downlink peak transmission rate of 100 megabits per second or higher, and an uplink peak transmission rate of 50 megabits per second or higher. The Third Generation Partnership Project (3GPP), which is an international standardization organization, has started developing LTE-Advanced (LTE-A), which is a standard for a mobile communication system based on the LTE, to achieve communications at even higher rates.
The LTE and the LTE-A both specify two different data reception scheduling algorithms, the dynamic scheduling (DS) and the semi-persistent scheduling (SPS). In the DS designates radio resources are indicated by a physical downlink control channel (PDCCH) to both uplink communications and downlink communications, and a physical downlink shared channel (PDSCH) representing data is transmitted using the indicated parameters.
In the SPS, a predetermined period having a particular transmission interval is notified in advance, before actual communications take place. When a communication using the SPS (hereinafter, referred to as an “SPS communication”) is to take place, the base station initiates the communication by transmitting an activation command on the PDCCH, to notify which radio resources are to be used continuously to a mobile station. For example, for a VoIP communication in which a first transmission of data occurs in every 20 milliseconds, a transmission interval of 20 milliseconds is signaled via radio resource control (RRC). When the actual communication is then to take place, a PDCCH activation command is used to indicate which radio resources are to be used, and an SPS communication at a 20 millisecond interval is initiated from that subframe. Because a transmission interval and radio resources are notified in advance, it is not necessary to transmit a downlink (DL) assignment and an uplink (UL) assignment (UL grant) over the PDCCH at each of the SPS intervals. Therefore, with the SPS communication, when the data packet size is small, only the data needs to be transmitted via the PDSCH, and no PDCCH is associated with the PDSCH that are small data. Therefore, the PDCCH signaling overhead can be reduced. The DS is used in data retransmissions, by contrast. When the SPS communication is completed, the base station transmits a release command via a PDCCH to the mobile station, or performs null transmissions for the number of times specified in a parameter called implicitReleaseAfter, to end the execution of the SPS communication. Specifically, the allocated radio resources are no longer used, while the SPS communication interval is still maintained. When an SPS communication is to be restarted, an activation command via a PDCCH is transmitted again to indicate the radio resources to be used. An SPS-like communication can be also achieved with the DS. For example, an SPS-like communication is achieved by causing a base station to perform a DS-based communication at an interval of 20 milliseconds. However, when the SPS-like communication is performed using the DS, the PDCCH signaling overhead is increased.
Japanese Laid-open Patent Publication No. 2009-165131 discloses a conventional technology that controls whether the radio resources are kept active or caused to transit to sleep after a scheduled period is completed based on the presence of transmission data when signals are received intermittently, e.g., in the case of SPS communications. Japanese National Publication of International Patent Application No. 2010-518765 discloses a technology that recovers synchronization when the timing of intermittent receptions between a base station and a mobile station are desynchronized, by signaling desynchronized discontinuous reception (DRX) timing. Other examples of related art are disclosed in: 3GPP TR 36.913, “Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced)”, V9.0.0, Release 9, December 2009; 3GPP TR36.912, “Feasibility study for further advancements for E-UTRA (LTE-Advanced)”, V9.3.0, Release 9, June 2010; 3GPP TS36.300, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)”, V10.4.0, Release 10, June 2011; and 3GPP TS36.321, “Medium Access Control (MAC) protocol specification”, V10.2.0, Release 10, June 2011.
However, when SPS communications at different communication intervals are configured to communications between a base station and a mobile station, two of the SPS communications might occur at the same timing. To address this issue, the LTE and the LTE-A specify that only one piece of data, more exactly, only one transport block can be transmitted within one subframe, unless multiple input multiple output (MIMO) transmissions are configured. Therefore, there is a possibility that one of the SPS communications might not be able to be continued.
When multiple SPS communications occur at the same timing even with the conventional technology for controlling whether the radio resources are to be kept active or caused to transit to sleep after a scheduled period is completed, or with the conventional technology for recovering synchronization, the SPS communications might not be able to be continued.